I had a similar issue, but I fixed it thanks to this reply on Reddit- seems like the Roblox client could be mishandling it. https://www.reddit.com/r/RobloxHelp/comments/1dnju0b/comment/m8g6nse
edit: in case the link gets deleted, here’s the content:
u/CaptainPross:
MY SOLUTION: I had this exact issue and i found out its a windows task called Mobile Devices when its running roblox will freeze on join and on respawn when the task is disabled there is no freezing at all. You can disable it in task manager and on startup apps and restart ur pc and hopefully the problem is solved.