What is the problem?
The waypoints of the flight are stored on the FMU SD card and when this slowdown in reading occurs, the drone keeps using the current waypoint (position) and then loops around it. It does not exactly loiter around (unless it's a loiter waypoint), as it tries to fly exactly through it.
What is the behaviour the customer will face?
The drone looks like having a weird flight behaviour with kind of aggressive turns going left or right. Or it can keep hovering before forward transition, because it is unable to read out the waypoint with the command to transition in time.
Solution
This is a known problem and will be generally improved on gen2 by using higher quality SD card.
For v3 drones, the customer should make sure to update the drone to v2.0 or higher. With v2.0 a short-term fix was introduced that reduces the amount of logging on the FMU sd card.
If that does not resolve the issue, they should replace the FMU SD card. The current FMU SD card is a micro SD card 16GB, Sandisk, Class 10. If the customer buys the replacement FMU SD card himself/herself, they need to format the SD card to FAT 32. Make sure that they are aware that the SD card should have a maximum capacity of 32GB, otherwise it cannot be formatted to FAT 32 format.
Replacement of SD card is required, formatting alone does NOT help!
The customer can keep flying with the current FMU SD card. It is safe, but it depends on the area they fly in. The drone may be looping around any of the waypoints at that waypoint's altitude. So, if they are flying in steep terrain, it can get potentially unsafe. It could also potentially loop then when coming back at the transition altitude - however, if they RTH before the end of the survey, it will be fine as RTH is stored internally in flash memory.