‘more rapidly‘ is an example of an adverb phrase. In this example, ‘rapidly‘ is the head of the phrase.
B2 point 55 in the English Grammar Profile under the category of adverbs is defined:
phrases that modify with an adverb and comparative structure.
What is particularly noticeable about this structure is that it attracts higher level vocabulary (as noted in EVP):
B1: accurately, frequently, heavily, seriously
B2: closely, deeply, effectively, efficiently, rapidly, readily, strongly
These adverbs would make a good set of vocabulary to pre-teach or check before going into this grammar structure.
The 20 most common ‘_RGR _RR than‘ in the iWeb corpus are:
1 MORE OFTEN THAN 75765
(this fixed phrase usually is connected in meaning to ‘most of the time’)
1 NOT (XX ) 53458
2 FAR (RG ) 1633
3 HAPPENS (VVZ ) 1527
4 USUAL (JJ ) 645
5 OCCURS (VVZ ) 221
2 MORE SO THAN 32118
3 MORE QUICKLY THAN 13506
4 MORE FREQUENTLY THAN 8520
Such things happen more frequently than one might expect.
5 MORE EASILY THAN 6437
6 MORE SLOWLY THAN 5440
7 MORE RAPIDLY THAN 4711
1 MUCH (RR ) 533
2 FAR (RG ) 272
3 EVER (RR ) 254
4 NORMAL (JJ ) 141
5 GROWING (VVG ) 134
8 MORE EFFECTIVELY THAN 4693
9 MORE EFFICIENTLY THAN 4085
10 LESS OFTEN THAN 2282
Less often than you might think.
11 LESS FREQUENTLY THAN 2272
1 EVERY (AT1 ) 239
2 ONCE (RR ) 193
3 FAR (RG ) 144
4 ANNUALLY (RR ) 79
5 OCCUR (VV0 ) 77
12 MORE CLEARLY THAN 1964
13 MORE READILY THAN 1850
14 LESS SO THAN 1754
15 MORE SERIOUSLY THAN 1744
16 MORE STRONGLY THAN 1610
17 MORE CLOSELY THAN 1603
18 MORE ACCURATELY THAN 1574
19 MORE HEAVILY THAN 1090
20 MORE DEEPLY THAN 1082