delay in arrayList processing vs String processing - arraylist

Question

Hello I have a question concerning data processing in groovy.
I have a groovy step in which I call a method that gives me a list of json in return, then I parse it to test a value
here is the code :
props = get_device_properties(device_id)
log.info "props = $props"
log.info "size " + props.size()
log.info "class " + props.class
//props = jsonSlurper.parseText(props)
log.info "***************************** t1"
props.find(){
log.info it.name
// check the field songTitle
if (it.name == "songTitle")
{
// first check media
if(init_correct == false)
{
log.info "init = " + it.value
if (it.value == "UNKNOWN")
found == true
log.info "***************************** t2"
here is a log of the process :
Tue Jan 28 11:34:56 CET 2020: INFO: props = [[satisfied:true, href:...
Tue Jan 28 11:35:03 CET 2020: INFO: size 31
Tue Jan 28 11:35:03 CET 2020: INFO: class class java.util.ArrayList
Tue Jan 28 11:35:03 CET 2020: INFO: ***************************** t1
Tue Jan 28 11:35:03 CET 2020: INFO: songTitle
Tue Jan 28 11:35:03 CET 2020: INFO: init = Sad But True
Tue Jan 28 11:35:03 CET 2020: INFO: ***************************** t2
If I parse the map directly there is a gap of time between method return and start of the next processing :
t2 - t1 is around 7s
If 'get_device_properties' returns a String instead of the arrayList and I do a parseText (uncomment the 5th line) on this string, I don't have the extra processing time before I can parse the map
Tue Jan 28 11:35:39 CET 2020: INFO: props = [{"satisfied":true,"href":...
Tue Jan 28 11:35:39 CET 2020: INFO: size 25138
Tue Jan 28 11:35:39 CET 2020: INFO: class class java.lang.String
Tue Jan 28 11:35:39 CET 2020: INFO: ***************************** t1
Tue Jan 28 11:35:39 CET 2020: INFO: songTitle
Tue Jan 28 11:35:39 CET 2020: INFO: init = Sad But True
Tue Jan 28 11:35:39 CET 2020: INFO: ***************************** t2
t2 - t1 = 0
can anyone help me to understand why this happens ?
indeed, I usually tend to use the most straightforward way to implement my script ...