Is there some kind of limits, or recommended number of cloud apps that we should install on the Jira cloud instance, that are custom developed, not from the Marketplace? And also can be there some kind of a network traffic that can slow down the Jira app?
No. Atlassian has no prescribed limit on apps. For the most part, the performance of Jira Cloud is not directly affected by the number of apps since both Connect & Forge make good use of asynchronous calls. The exceptions mainly come from use of the very old Universal Plugin Manager (UPM). For example, my own developer instance has ~100 apps installed at any given time, as a mix of personal dev projects and those of partners with whom I’ve collaborated. With very little data, most Jira screens are as snappy as a fresh instance, with the major exception being the “Manage apps” screen, which can take minutes to load.
No, not really. A slow connection to the app might make parts of Jira load slow; specifically, the parts that are provided by the app itself. Or, may cause some app functions to feel sluggish. For example, any app responding to events (either Forge events or Connect webhooks), might not get the event or respond before the user tries to do the next thing. Probably the most impactful traffic would be “expensive” API requests. For example, complex JQL is more-or-less translated to complex SQL on the back-end, which could cause short bursts of performance degradation, but the cost-based rate limits are already a guard against that causing too much degradation over time. All that to say, apps do have an impact, but more for the perception of users than on the observable performance metrics of Jira itself.