Yes you are correct; If you have a slow time base of +100ms/div you won't see the maths channel, until the trace has reached the end of the waveform buffer. This because the maths channels are done after waveform buffer has been captured.
In faster time base(50ms/div or less)the scope is put into block capture. The scope captures data to its internal memory and is then transferred to the PC, and the whole waveform buffer is displayed in picoscope. So any maths channels are displayed at the same time.
In slower time base of +100ms/div the scope operating in streaming mode, the scope captures data and streams it directly to the PC to display. And you can see trace being drawn across the screen.
You can increase this transmission point of the scope going into stream mode, in the PicoScope settings.
To change this;
Go to the 'Tools' menu - 'Preferences' - 'Sampling' tab - In the 'collection time' set this to the max. time/division, that the scope will switch into streaming mode.
i.e. 2s/div (So if you set this to 2s/div the scope still operate in block mode set at 1s/div or faster. )
So you have wait for whole buffer to be sampled, before its displayed on screen. (So in block at 1s/div the trace will be upto1 second behind the actual value being captured on the scope)
For more information please see our website A to Z under- Block capture mode and Streaming mode https://www.picotech.com/library/a-to-z
You can also increase the number of waveform buffers from the default of 32 upto 10000, to get the total capture time you need.
Goto 'Tools' menu -> 'Preferences' -> 'General' tab -> 'waveform buffer' section
You may want use alarms in picoscope to stop the capture once all the buffers are full, otherwise picoscope will just continue capturing, keeping the most recent number of buffers.
Alarms see page 36- https://www.picotech.com/download/manuals/PicoScope6UsersGuide.pdf