Is download speed affected by cellular activity?

Off hand what you’ve described is a result of de-prioritization.

T-Mobile Home Internet customers receive the same network prioritization as Heavy Data Users (specifically the lowest priority versus other traffic). As I result, you’ll likely experience lower speeds during peak utilization periods (congestion).

See the Open Internet link appearing at the bottom of this page.

This can be from a couple of items.

1. As mentioned - deprioritization this hits some areas worse than others.   I’ve been lucky enough to not really have this in my area (suburbia)
2. Overall signal quality/congestion.  At times, your device may switch from 5G n41 + LTE to 5G n71 + LTE to … LTE only.  When this occurs for me, +400/20Mbps → 170/70Mbps, and it is typically due to SINR/RSRP values.  If your SINR is low, its either congestion (spectrum) or interference (objects).   When I go below 8 on SINR for n41, it often drops to n71.  SINR of 15 or better, and I’m rocking.  I’m only ~1200’ away from T-Mobile tower, but there’s a building in between blocking

Man that all sounds greek to me.  I’m guessing I can see SINR and RSRP under Advanced Cellular Metrics?  I don’t follow about 5G n41 LTE etc.  Under Advanced Cellular Metrics the top bar has LTE on left and 5G or right.  When LTE is underlined I have SINR 3.1 and RSRP -106.  When I underline 5G SINR is 12 and RSRP -99.  Status shows POOR for all 4 values for those.  However right now I am getting download/upload 204/5 according to Ookla SpeedTest.  Everything I do seems to work fine at those speeds.

ok - on 4G LTE  and 5G items, you’ll want to also look for ‘band’

If your band shows 41 on 5G, the’s good, and may also explain why upload is slow.  I do suspect that it is 5G n71, and LTE is B2 or B66 (based on RSRP readings).

For 3 miles, that’s not a horrible signal, and is probably a clear line of site.   Its possibly a combination of:

1. Deprioritization
2. More ‘users’ utilizing them spectrum , causing poorer values for SINR/RSRP.  Eg.  I can see during the day SINR go from 15 down to 8 on 5G and performance go from ~500Mbps down to 380.

I’m suspecting the 2nd more than the 1st, as true deprioritization would make many services almost unusable.

Now I suspect T-Mobile is pulling a fast one.  For the first month (during my 15 day trial) I often saw download speeds of 250+ but now that I’m in the second month every time it’s less than 100. Usually around 80.  Even though I’m usually seeing 3 bars - GOOD connection.  Is T-Mobile throttling me now that I’ve committed to using them?