When discussing RHEL-108115 it looks possible that issue is exacerbated by the fact that lftp doesn't disable the Nagle algorithm (setting the TCP_NODELAY socket option).

For at least the FTP data connection and probably the control connection this is certainly appropriate and should be the default to avoid surprises.

Contrast with curl - a much more widely used FTP client than lftp - which is disabling Nagle for both the control and data connections.

$ strace -econnect,setsockopt curl ftp://ftp.mirrorservice.org/
setsockopt(4, SOL_TCP, TCP_NODELAY, [1], 4) = 0
setsockopt(4, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0
setsockopt(4, SOL_TCP, TCP_KEEPIDLE, [60], 4) = 0
setsockopt(4, SOL_TCP, TCP_KEEPINTVL, [60], 4) = 0
setsockopt(4, SOL_TCP, TCP_KEEPCNT, [9], 4) = 0
connect(4, {sa_family=AF_INET6, sin6_port=htons(21), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "2001:630:341:12::184", &sin6_addr), sin6_scope_id=0}, 28) = -1 EINPROGRESS (Operation now in progress)
setsockopt(5, SOL_TCP, TCP_NODELAY, [1], 4) = 0
setsockopt(5, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0
setsockopt(5, SOL_TCP, TCP_KEEPIDLE, [60], 4) = 0
setsockopt(5, SOL_TCP, TCP_KEEPINTVL, [60], 4) = 0
setsockopt(5, SOL_TCP, TCP_KEEPCNT, [9], 4) = 0
connect(5, {sa_family=AF_INET6, sin6_port=htons(59061), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "2001:630:341:12::184", &sin6_addr), sin6_scope_id=0}, 28) = -1 EINPROGRESS (Operation now in progress)
drwxr-xr-x    2 ftp      ftp          4096 Aug 02  2023 pub
drwxr-xr-x  142 ftp      ftp          4096 Mar 06 11:00 sites