I probably should have said AAL5 over ATM, or only AAL5, but that piece is not particularly important.
As far as I can tell, this "serialisation delay" is simply the time taken to transfer X number of bits at a speed of Y bits per second.
64 bytes and a rate of 64kbps
64*8 = 512 bits
512/64000 = 0.008 seconds = 8ms
The small cell size of ATM in general may well be for all the benefits of the small cell size, but specifically, for the ADSL link where the ATM cells are only going between the modem and DSLAM, I think most of the general features of ATM networks aren't used. Nothing using your Internet connection would even be aware that it's using ATM cells in that section of the link.
1500 bytes and a rate of 780kbps (my current upstream speed)
(1500*8)/780000 = 0.0153846153 seconds, or about 15ms
$ ping -c 10 -s 106 212.159.6.9
PING 212.159.6.9 (212.159.6.9) 106(134) bytes of data.
114 bytes from 212.159.6.9: icmp_seq=1 ttl=59 time=25.8 ms
114 bytes from 212.159.6.9: icmp_seq=2 ttl=59 time=25.5 ms
114 bytes from 212.159.6.9: icmp_seq=3 ttl=59 time=25.9 ms
114 bytes from 212.159.6.9: icmp_seq=4 ttl=59 time=26.2 ms
114 bytes from 212.159.6.9: icmp_seq=5 ttl=59 time=25.2 ms
114 bytes from 212.159.6.9: icmp_seq=6 ttl=59 time=25.1 ms
114 bytes from 212.159.6.9: icmp_seq=7 ttl=59 time=25.4 ms
114 bytes from 212.159.6.9: icmp_seq=8 ttl=59 time=24.8 ms
114 bytes from 212.159.6.9: icmp_seq=9 ttl=59 time=25.0 ms
114 bytes from 212.159.6.9: icmp_seq=10 ttl=59 time=25.2 ms
--- 212.159.6.9 ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 9011ms
rtt min/avg/max/mdev = 24.807/25.461/26.219/0.471 ms
$ ping -c 10 -s 1450 212.159.6.9
PING 212.159.6.9 (212.159.6.9) 1450(1478) bytes of data.
1458 bytes from 212.159.6.9: icmp_seq=1 ttl=59 time=44.9 ms
1458 bytes from 212.159.6.9: icmp_seq=2 ttl=59 time=45.3 ms
1458 bytes from 212.159.6.9: icmp_seq=3 ttl=59 time=44.7 ms
1458 bytes from 212.159.6.9: icmp_seq=4 ttl=59 time=44.2 ms
1458 bytes from 212.159.6.9: icmp_seq=5 ttl=59 time=44.5 ms
1458 bytes from 212.159.6.9: icmp_seq=6 ttl=59 time=44.0 ms
1458 bytes from 212.159.6.9: icmp_seq=7 ttl=59 time=44.3 ms
1458 bytes from 212.159.6.9: icmp_seq=8 ttl=59 time=43.9 ms
1458 bytes from 212.159.6.9: icmp_seq=9 ttl=59 time=44.0 ms
1458 bytes from 212.159.6.9: icmp_seq=10 ttl=59 time=44.3 ms
--- 212.159.6.9 ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 9012ms
rtt min/avg/max/mdev = 43.969/44.450/45.319/0.489 ms
Of course the ping times are still higher with larger packets, they will have been split up into ATM cells, but it still needs all the ATM cells to arrive at the other end before it can re-assemble the full packet. So I don't really think these small ATM cells have improved anything.
ADSL1 only had the choice of ATM or STM. But I can't really see any disadvantages of using PTM 64/65 octet encapsulation with ADSL2 for any amount of bandwidth, large or small.