Very low water pressure in bathroom

Your current shower flow rate may actually be the modern standard for water saving. The reduction in flow may be due to new piping being smaller or may be due to the shower head. If the latter is the case there are easy remedies.

Did you get a new shower head in the renovation? New heads in general have a much lower flow rate than older ones. The highest flow modern heads have a max flow of 2.5 gal/min. There are lower flow (water saving) ones with 2 gal/min, 1.5 gal/min or even 1 gal/min. The flow rate for a shower head is usually printed on the end that you can see. It will be given in gallons per minute (gal/min or GPM) and in liters per min (L/MIN). What flow is given on your head?

You can determine the flow in gal/min. If you have a tub/shower, you can determine the flow rate through the valving by using the spigot to fill a 2 gal bucket or a 2 qt pitcher (1/2 gal).

If you have a shower only, turn on the shower to full volume and then thrust a 2 qt pitcher under the head and measure the time in seconds required to fill it. If in the renovation a 1.5 gal/min head was installed, then it would take 20 seconds to fill a 2-qt pitcher.

Then remove the shower head and measure the flow rate. Without a head I would expect a flow rate of 4 gal/min or more, but modern PEX plumbing may give less.

EDIT

Sample calculation of flow rate. It takes 21 sec to fill up a 2-qt pitcher:

(0.5 gal / 21 s) x 60 s/min = 1.4 gal/min