Environment:
Jira Software Server 8. 6.1
To reproduce:
Import @atlaskit/navigation-next, or the code in attachment (extract from @atlaskit/navigation-next node_modules) on a js page.
Expected result
The js file is downloaded with no notable delay.
Actual result
It takes 10-20 seconds waiting time before it is downloaded by the browser. Jconsole confirms that processing of the requests by the Jira Server takes 19 seconds.
Code that is causing the performance issues
After trial & error, I’ve isolated the part of the code that is causing the performance issue.! 
extract-atlaskit-navigation-next-perf-issue.js.pdf (283.2 KB)
The sourceMappingURL base64 encodings cause this, for example:
/*!*****************************************************************************************************!*\
!*** Extract ./node_modules/@atlaskit/navigation-next/dist/esm/components/presentational/Switcher/index.js ***!
\*****************************************************************************************************/
var a = "/*# sourceMappingURL=data:application/json;charset=utf-80;base64,{"version":3,"sources":["../../../../../src/components/presentational/Switcher/index.js"],"names":[],"mappings":"AA6EI","file":"../../../../../src/components/presentational/Switcher/index.js","sourcesContent":["// @flow\nimport React, { PureComponent, cloneElement, type ElementRef } from 'react';\nimport NodeResolver from 'react-node-resolver';\nimport shallowEqualObjects from 'shallow-equal/objects';\nimport { components, PopupSelect, mergeStyles } from '@atlaskit/select';\nimport { gridSize as gridSizeFn } from '@atlaskit/theme/constants';\nimport { N30, B50, N40A, N200, B300 } from '@atlaskit/theme/colors';\nimport AddIcon from '@atlaskit/icon/glyph/add';\n\nimport {\n  type SwitcherState,\n  type SwitcherProps,\n  type SwitcherBaseProps,\n  type SelectStyles,\n  type OptionType,\n} from './types';\nimport Option from './Option';\nimport { UIControllerSubscriber } from '../../../ui-controller';\nimport { CONTENT_NAV_WIDTH } from '../../../common/constants';\n\nconst gridSize = gridSizeFn();\n\nconst defaultStyles = {\n  option: (provided, { isActive, isFocused }) => {\n    return {\n      ...provided,\n      alignItems: 'center',\n      border: 'none',\n      backgroundColor: isFocused ? N30 : 'transparent',\n      boxSizing: 'border-box',\n      color: 'inherit',\n      cursor: 'default',\n      display: 'flex',\n      flexShrink: 0,\n      fontSize: 'inherit',\n      height: gridSize * 6,\n      outline: 'none',\n      paddingRight: gridSize,\n      paddingLeft: gridSize,\n      textAlign: 'left',\n      textDecoration: 'none',\n      width: '100%',\n      ...(isActive && { backgroundColor: B50 }),\n    };\n  },\n};\n\n// ==============================\n// Custom Functions\n// ==============================\n\nexport const createStyles = (styles: SelectStyles = {}): Object =>\n  mergeStyles(defaultStyles, styles);\n\nexport const filterOption = ({ data }: { data: OptionType }, input: string) =>\n  data.text.toLowerCase().includes(input.toLowerCase());\n\nexport const isOptionSelected = (\n  option: OptionType,\n  selected: Array<OptionType> | void,\n) => {\n  if (!selected || !selected.length) return false;\n  return option.id === selected[0].id;\n};\n\nexport const getOptionValue = (option: OptionType) => option.id;\n\n// ==============================\n// Custom Components\n// ==============================\n\nexport const Control = ({\n  innerProps: { innerRef, ...innerProps },\n  ...props\n}: *) => (\n  <div\n    ref={innerRef}\n    css={{\n      boxShadow: `0 2px 0 ${N40A}`,\n      padding: gridSize,\n      position: 'relative',\n    }}\n  >\n    <components.Control {...props} innerProps={innerProps} />\n  </div>\n);\nexport const Footer = ({ text, onClick }: *) => (\n  <button\n    css={{\n      background: 0,\n      border: 0,\n      boxShadow: `0 -2px 0 ${N40A}`,\n      boxSizing: 'border-box',\n      color: N200,\n      cursor: 'pointer',\n      alignItems: 'center',\n      display: 'flex',\n      fontSize: 'inherit',\n      padding: `${gridSize * 1.5}px ${gridSize}px`,\n      position: 'relative',\n      textAlign: 'left',\n      width: '100%',\n\n      ':hover, :focus': {\n        color: B300,\n        outline: 0,\n        textDecoration: 'underline',\n      },\n    }}\n    onClick={onClick}\n  >\n    <AddIcon label=\"Add icon\" size=\"small\" />\n    <span css={{ marginLeft: gridSize }}>{text}</span>\n  </button>\n);\n\nconst defaultComponents = { Control, Option };\nconst isEmpty = obj => Object.keys(obj).length === 0;\n\n// ==============================\n// Class\n// ==============================\n\nclass Switcher extends PureComponent<SwitcherProps, SwitcherState> {\n  state = {\n    mergedComponents: defaultComponents,\n  };\n\n  selectRef = React.createRef();\n\n  targetRef: ElementRef<*>;\n\n  targetWidth = 0;\n\n  static defaultProps = {\n    closeMenuOnCreate: true,\n    components: {},\n    navWidth: CONTENT_NAV_WIDTH,\n    isNavResizing: false,\n  };\n\n  static getDerivedStateFromProps(props: SwitcherProps, state: SwitcherState) {\n    const newState = {};\n\n    // Merge consumer and default components\n    const mergedComponents = { ...defaultComponents, ...props.components };\n    if (!shallowEqualObjects(mergedComponents, state.mergedComponents)) {\n      newState.mergedComponents = mergedComponents;\n    }\n\n    if (!isEmpty(newState)) return newState;\n\n    return null;\n  }\n\n  componentDidUpdate({ isNavResizing }: SwitcherProps) {\n    if (\n      isNavResizing &&\n      this.selectRef.current &&\n      this.selectRef.current.state.isOpen\n    ) {\n      this.selectRef.current.close();\n    }\n  }\n\n  resolveTargetRef = (popupRef: ElementRef<*>) => (ref: HTMLElement) => {\n    // avoid thrashing fn calls\n    if (!this.targetRef && popupRef && ref) {\n      this.targetRef = ref;\n      popupRef(ref);\n    }\n  };\n\n  getFooter = () => {\n    const { closeMenuOnCreate, create, footer } = this.props;\n\n    if (footer) return footer;\n    if (!create) return null;\n\n    let { onClick } = create;\n    if (closeMenuOnCreate) {\n      onClick = e => {\n        if (this.selectRef.current) {\n          this.selectRef.current.close();\n        }\n        create.onClick(e);\n      };\n    }\n\n    return <Footer text={create.text} onClick={onClick} />;\n  };\n\n  render() {\n    const { create, options, target, ...props } = this.props;\n    const { mergedComponents } = this.state;\n    const targetWidth = this.props.navWidth - gridSize * 2;\n\n    return (\n      <PopupSelect\n        ref={this.selectRef}\n        filterOption={filterOption}\n        isOptionSelected={isOptionSelected}\n        footer={this.getFooter()}\n        getOptionValue={getOptionValue}\n        options={options}\n        maxMenuWidth={targetWidth}\n        minMenuWidth={targetWidth}\n        target={({ ref, isOpen }) => (\n          <NodeResolver innerRef={this.resolveTargetRef(ref)}>\n            {cloneElement(target, { isSelected: isOpen })}\n          </NodeResolver>\n        )}\n        {...props}\n        styles={createStyles(this.props.styles)}\n        components={mergedComponents}\n      />\n    );\n  }\n}\n\nexport { Switcher as BaseSwitcher };\n\nexport default (props: SwitcherBaseProps) => (\n  <UIControllerSubscriber>\n    {({ state }) => (\n      <Switcher\n        navWidth={state.productNavWidth}\n        isNavResizing={state.isResizing}\n        {...props}\n      />\n    )}\n  </UIControllerSubscriber>\n);\n"]} */";