Hi Amnon,
Are you using the recommended scripts to program the ADV7188?
Can you try connecting your camera sources to an ADV7188 eval board to determine if the lock time is okay in this configuration?
The ADV7188 should not take 5 to 20 seconds to lock to a standard PAL or NTSC signal.
Can you probe the CVBS inputs before the AC coupling cap an ensure the signal appears to be okay at the time of the switch between sources?
Can you also monitor the status register 1 (Addr 0x10) "IN_LOCK" and "FSC_LOCK" bits. These will indicate when the ADV7188 has locked to the H sync and FSC.
Regards,
Derek