As of summer '19, OnDemand remains useless.

As a simulator, its particularly useless because the only benefit to be had from sim trading: is conditioning oneself to the mechanics of trading. OnDemand produces a noticeably cheesy feel. (sim trading can't be a test of one's decision making ability because there's no risk (emotion) involved).

I looked at it again a few months ago and found that about half of the last 8 equity contracts are unavailable, so if you're looking to trade a particular day type, your choices are slim.

Another way to speed up your TOS platform is to clear the cache file. In TOS, go to Help, System, then click Collect Garbage. You should see the Free memory figure increase a bit.

when using OnDemand, close all watch list, close as many as study that you don't need.
Sometimes click the pause button on the OnDemand toolbar to give time for the app response to the data.

I have a decent computer, (i7-9700K "4.9GHz, 32GB ram, 2080GTX) and I have rarely any lag unless I speed up all 8 charts at 3x speed (then sometimes it takes some time before the chart updates).
What I do however, is that since TOS can't utilize more than 2 cores on the computer it seems, I open 4 instances of TOS, each instance with 2 charts on. Then when I run on demand, it utilize 100% of my CPU, instead of max 20-30% when I have 8 charts with only one instance open with TOS.

This is probably a network congestion issue with the OnDemand server. I doubt thinkorswim originally designed it to handle heaps of Ameritrade users.

If you watch the thinkorswim program on your computer’s activity monitor you’d see that it downloads a small data packet (historical market data) from the internet every 5 mins or so. When it says “prebuffering...” the application is actually idle with no disk activity until it receives that data from the server, then it starts going again.

So not really an issue with your system. The issue is more likely there are thousands of traders asking for data every five mins so you have to wait in line.