As of late, it seems that any thread over ~40 or so emails takes upwards of 15 seconds to load, is quite laggy, and causes instability in the browser even after leaving the long thread. Loading attachments from such threads also seems slower than <5 message threads.
Gmail is incapable of handling threads at all, as a matter of design: <a href="https://lwn.net/Articles/837960/" rel="nofollow">https://lwn.net/Articles/837960/</a>