Thanks to you both for the help. The results of fixdisk did show a few errors corrected on sda2.
So far it seems to be behaving ok, just ridiculously slow trying to access content of folders, "processing...".
In case it's of interest:
Mon Jun 21 10:50:57 BST 2021: Checking partition /dev/sda2...
e2fsck 1.42.13 (17-May-2015)
Pass 1: Checking inodes, blocks, and sizes
Inode 268175, i_blocks is 456912, should be 128912. Fix? yes
Pass 1: Memory used: 1780k/468k (912k/869k), time: 6589.97/4027.26/54.08
Pass 1: I/O read: 1963MB, write: 1MB, rate: 0.30MB/s
Pass 2: Checking directory structure
Pass 2: Memory used: 1780k/468k (1189k/592k), time: 4.18/ 1.05/ 0.14
Pass 2: I/O read: 5MB, write: 0MB, rate: 1.20MB/s
Pass 3: Checking directory connectivity
Peak memory: Memory used: 1780k/468k (1189k/592k), time: 6598.66/4032.43/54.27
Pass 3A: Memory used: 1780k/468k (1202k/579k), time: 0.00/ 0.00/ 0.00
Pass 3A: I/O read: 0MB, write: 0MB, rate: 0.00MB/s
Pass 3: Memory used: 1780k/468k (1183k/598k), time: 0.06/ 0.05/ 0.00
Pass 3: I/O read: 1MB, write: 0MB, rate: 16.71MB/s
Pass 4: Checking reference counts
Pass 4: Memory used: 1780k/468k (500k/1281k), time: 3.56/ 3.46/ 0.10
Pass 4: I/O read: 0MB, write: 0MB, rate: 0.00MB/s
Pass 5: Checking group summary information
Block bitmap differences: +(98607534--98607535)
Fix? yes
Free blocks count wrong for group #3009 (24269, counted=24266).
Fix? yes
Free blocks count wrong for group #9278 (0, counted=29256).
Fix? yes
Free blocks count wrong for group #9279 (21002, counted=32746).
Fix? yes
Free blocks count wrong (99104649, counted=99145646).
Fix? yes
Pass 5: Memory used: 1780k/468k (487k/1294k), time: 165.19/27.84/ 4.17
Pass 5: I/O read: 116MB, write: 0MB, rate: 0.70MB/s
/dev/sda2: ***** FILE SYSTEM WAS MODIFIED *****
14320 inodes used (0.76%, out of 1896576)
2164 non-contiguous files (15.1%)
29 non-contiguous directories (0.2%)
# of inodes with ind/dind/tind blocks: 4595/3612/1
386346679 blocks used (79.58%, out of 485492325)
0 bad blocks
49 large files
12509 regular files
467 directories
0 character device files
0 block device files
0 fifos
2 links
1335 symbolic links (1335 fast symbolic links)
0 sockets
------------
14313 files
Memory used: 1780k/468k (487k/1294k), time: 6828.93/4064.26/59.27
I/O read: 2085MB, write: 59MB, rate: 0.31MB/s
Mon Jun 21 12:44:47 BST 2021
Removing extra swap space.
Finished
fix-disk: session terminated with exit status 0