I’ve experienced this only since 3.1, I did mention it a couple of times during beta but it was never addressed (or reproduced?).
If I open renoise, double click on a sample to load it in to a sample slot renoise will freeze for a moment. The amount of time it freezes for depends on the length of the sample, things like full vocal stems a few minutes long cause a freeze of 10 seconds or so. This only happens with the first sample I load, all samples after load instantaneously. Sample preview seems to have no effect on this behaviour one way or the other. There is also another condition, it has to be a new instance of renoise. If I load renoise, load a sample, get the freeze, then open a new project the freeze doesn’t happen again. However, restarting renoise and loading a sample will cause the freeze again.
This is linux 64 bit, renoise 3.1. The log shows no event or information for the freezing.
edit: happens with instruments as well.