iPhone XS bests iPhone XR in cellular signal strength thanks to better antennas
Even though they share the same Intel modem, the iPhone XS gets definitively better signal strength than the iPhone XR, according to test results shared on Monday.

The iPhone XS and XS Max on the left, and the iPhone XR on the right.
Download speeds achieved with the iPhone XR are similar to those of 2017's iPhone X, but the iPhone XS is nearly twice as fast as both, PC Mag said, citing research done in cooperation with Cellular Insights and Rohde & Schwarz. The testing was done with a single 20-megaherz carrier of LTE Band 4, owing to time restrictions.

Image Credit: PC Mag
The difference is connected to the XS's support for 4x4 MIMO, whereas the X and XR only have 2x2. The XS achieves not just a 400-megabit top speed on Band 4, but better data under weak signal conditions.
When connecting to a 2x2 MIMO cell site, the XS still offers faster downloads for longer, for example holding onto the peak 200-megabit speed of 2x2 well after the X and XR have begun slowing down. The XS is simply superior in areas with weak LTE, PC Mag concluded.

Image Credit: PC Mag
On compatible LTE networks the XS can theoretically achieve gigabit speeds. That's unlikely in the real world, since most cell sites have some degree of network congestion and signal interference.

The iPhone XS and XS Max on the left, and the iPhone XR on the right.
Download speeds achieved with the iPhone XR are similar to those of 2017's iPhone X, but the iPhone XS is nearly twice as fast as both, PC Mag said, citing research done in cooperation with Cellular Insights and Rohde & Schwarz. The testing was done with a single 20-megaherz carrier of LTE Band 4, owing to time restrictions.

Image Credit: PC Mag
The difference is connected to the XS's support for 4x4 MIMO, whereas the X and XR only have 2x2. The XS achieves not just a 400-megabit top speed on Band 4, but better data under weak signal conditions.
When connecting to a 2x2 MIMO cell site, the XS still offers faster downloads for longer, for example holding onto the peak 200-megabit speed of 2x2 well after the X and XR have begun slowing down. The XS is simply superior in areas with weak LTE, PC Mag concluded.

Image Credit: PC Mag
On compatible LTE networks the XS can theoretically achieve gigabit speeds. That's unlikely in the real world, since most cell sites have some degree of network congestion and signal interference.
Comments
Would the larger body of the XS Max mean the antenna are larger? And would that possibly lead to even better low signal performance? Or is that just wishful thinking on my part...
It would have taken hours on a sub-100 Mbps connection.
Uploading images from an on-site photoshoot to clients, designers and/or retouchers is another place I use a fast data connection. Often you do not have on-site data available due to IT policy or just lack of infrastructure. Dropbox & super-fast cellular data really come through in these situations.
Any idea what OS PC Mag was running?