This is a memory leak issue. Although we see the issue here in IE11, other browsers should theoretically have similar memory / performance issues on 3.5.z. I was able to reproduce the issue in oVirt 3.5.8, with 200 hosts, using IE11 and idling on the hosts tab for 15 minutes. Memory profile in IE showed a relatively high amount of memory leaking (1.4 GB in 9 minutes). I then mimicked that setup in 4.0-master, also with 200 hosts. In IE11 and idling for 30 minutes, I could not reproduce the issue. Memory profile showed way less memory burn than 3.5.8. I then applied a patch from Bug 1294678 to 3.5.8, thinking that that was such a large leak that maybe this patch alone would help. But I still got the error popup with that patch applied. Thus, I suspect there have been several frontend memory improvements between 3.5.8 and 4.0. I did not do any testing on 3.6. For reference, 3.6.5 has the patch from Bug 1294678 applied, and has other memory improvements. My recommendation for now would be to mark this as fixed in 4.0, and possibly fixed in 3.6.5 too.
We decided to test this out on 3.6.7. In 3.6.7, memory is leaking worse than in 4.0, but not as bad as in 3.5.8. In IE11, I idled for 30 minutes and did not receive the error popup. However, after 30 minutes, the performance of the UI was sluggish. 900MB of memory was used, which is high in my setup. Using Chrome 49 on the same 3.6.7 setup, and also idling for 30 minutes, I didn't receive an error popup, nor did the UI become sluggish. Based on this, I conclude that 4.0 has better memory usage and will work better for IE11 users.
Verified on 4.0.0-19 on Win7 64b with IE11
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://rhn.redhat.com/errata/RHEA-2016-1743.html